gistfile1.txt

$installer->addAttribute('catalog_product', "shirt_size", array(
    'type'       => 'int',
    'input'      => 'select',
    'label'      => 'Shirt Size',
    'sort_order' => 1000,
    'required'   => false,
    'global'     => Mage_Catalog_Model_Resource_Eav_Attribute::SCOPE_STORE,
    'backend'    => 'eav/entity_attribute_backend_array',
    'option'     => array (
        'values' => array(
            0 => 'Small',
            1 => 'Medium',
            2 => 'Large',
        )
    ),

));